A. De Giacomo

Heavy metal concentrations in soils as determined by laser-induced breakdown spectroscopy (LIBS), with special emphasis on chromium

Soil is unanimously considered as one of the most important sink of heavy metals released by human activities. Heavy metal analysis of natural and polluted soils is generally conducted by the use of atomic absorption spectroscopy (AAS) or inductively coupled plasma optical emission spectroscopy (ICP-OES) on adequately obtained soil extracts. Although in recent years the emergent technique of laser-induced breakdown spectroscopy (LIBS) has been applied widely and with increasing success for the qualitative and quantitative analyses of a number of heavy metals in soil matrices with relevant simplification of the conventional methodologies, the technique still requires further confirmation before it can be applied fully successfully in soil analyses. The main objective of this work was to demonstrate that new developments in LIBS technique are able to provide reliable qualitative and quantitative analytical evaluation of several heavy metals in soils, with special focus on the element chromium (Cr), and with reference to the concentrations measured by conventional ICP spectroscopy. The preliminary qualitative LIBS analysis of five soil samples and one sewage sludge sample has allowed the detection of a number of elements including Al, Ca, Cr, Cu, Fe, Mg, Mn, Pb, Si, Ti, V and Zn. Of these, a quantitative analysis was also possible for the elements Cr, Cu, Pb, V and Zn based on the obtained linearity of the calibration curves constructed for each heavy metal, i.e., the proportionality between the intensity of the LIBS emission peaks and the concentration of each heavy metal in the sample measured by ICP. In particular, a triplet of emission lines for Cr could be used for its quantitative measurement. The consistency of experiments made on various samples was supported by the same characteristics of the laser-induced plasma (LIP), i.e., the typical linear distribution confirming the existence of local thermodynamic equilibrium (LTE) condition, and similar excitation temperatures and comparable electron number density measured for all samples. An index of the anthropogenic contribution of Cr in polluted soils was calculated in comparison to a non-polluted reference soil. Thus, the intensity ratios of the emission lines of heavy metal can be used to detect in few minutes the polluted areas for which a more detailed sampling and analysis can be useful.

Experimental characterization of metallic titanium-laser induced plasma by time and space resolved optical emission spectroscopy

Abstract Time and space resolved optical emission spectroscopy has been successfully employed to investigate the evolution of the plasma produced by the interaction of UV laser beam with a metallic target of titanium at two different pressures (10 −5 and 3.4×10 −2 torr) and at distances up to 3 mm from the target. By time of flight measurements and Boltzmann plots both the dynamic and the kinetic aspects have been discussed. The quasi-equilibrium state of the laser-induced plasma has been established on the basis of the failure of Saha balance equation. The effect of three-body recombination on atomic titanium temporal distribution has been explained. Temporal evolution of electron number density, as determined by Stark effect, has been used for the estimation of the three-body recombination rate constant.

Optical emission spectroscopy and modeling of plasma produced by laser ablation of titanium oxides

Abstract In the present study, the time evolution of electron number density, of electron, atom and ion temperatures, of plasma produced by KrF excimer laser ablation of titanium dioxide and monoxide targets, are investigated by temporally and spatially resolved optical emission spectroscopy over a wide range of laser fluence from 1.7 to 6 J cm −2 , oxygen pressures of 10 −2 –10 −1 torr and in a vacuum. A state-to-state collisional radiative model is proposed for the first time to interpret the experimental results at a distance of 0.6 mm from the target surface, in vacuum and for a time delay from 100 to 300 ns from the beginning of the laser pulse. In particular, we concentrate our attention on problems concerning the existence of the local thermodynamic conditions in the laser-induced plasma and deviation from them, as observed in our experiment. The numerical model proposed for calculating the electron number density and the population densities of atoms and ions in excited states give good quantitative agreement with the experimental results of the optical emission spectroscopy measurements.

Spectroscopic investigation of the technique of plasma assisted pulsed laser deposition of titanium dioxide

Abstract The characterization of plasma assisted pulsed laser deposition (PA-PLD) of titanium dioxide with biased substrate is discussed. Both the stage of plasma expansion and deposition have been studied. Optical emission spectroscopy was employed to estimate laser-induced plasma parameters, while different techniques [optical microscopy, scanning electron microscopy (SEM), spectrophotometry, X-ray photoelectron spectrometry (XPS)] were used to characterize the film properties. It is shown that PA-PLD prevents contamination of the deposited films by particles ejected during the interaction of the KrF excimer laser radiation with the titanium dioxide targets. Investigation made on the film deposited by conventional PLD and PA-PLD, has shown that the PA-PLD technique allows to improve the quality of the deposited films for what concerns their stoichiometry, morphology and deposition rate.

Laser Ablation of Titanium Metallic Targets: Comparison Between Theory and Experiment

A model of plasma expansion with chemical kinetics has been developed and compared with the free-flow model describing the laser ablation plume of metallic titanium target. Optical emission spectroscopy has been used to obtain time of flight (TOF) spectra. The measured shift between Ti and Ti + TOF has been theoretically explained on the basis of the recombination process. Particular attention has been also focused on the dependence of TOF on some plume parameters such as initial speed, temperature, pressure, and plume extension.

Plasma-assisted pulsed laser deposition for the improvement of the film growth process

In this paper, results are reported on the characterization of thin films of titanium dioxide and chalcogenide glass doped with praseodymium by the technique of plasma-assisted pulsed laser deposition (PA-PLD) with biased substrate. This technique is shown to be able to prevent contamination of deposited films by particles ejected and to improve the pulsed laser deposition process for stoichiometry, morphology and optical properties of the films produced.

Perspective on the use of nanoparticles to improve LIBS analytical performance: nanoparticle enhanced laser induced breakdown spectroscopy (NELIBS)

In this paper, the new approach for Laser Induced Breakdown Spectroscopy (LIBS) based on nanoparticle deposition on the sample surface is reviewed from both fundamental and application points of view. The case of Nanoparticle-Enhanced LIBS (NELIBS) of metal samples is used for describing and discussing the main causes of the emission signal enhancement. A set of test cases is presented, which shows enhancements up to 1–2 orders of magnitude obtained using NELIBS with respect to LIBS. The feasibility and potential of NELIBS are also discussed for several analytical applications, including analysis of metallic samples, transparent samples and aqueous solutions.

The effect of oxygen rf discharge on pulsed laser deposition of oxide films

In this work final results on TiO2 film deposition by Plasma Assisted Pulsed Laser Deposition (PAPLD) with an rf biased substrate are presented. In previous work it has been shown that PAPLD is an improvement over conventional PLD for the elimination of particulates in high refractive index thin film deposition. This paper will give a comparison between conventional PLD and PAPLD on the stoichiometry, morphology, and optical properties of deposited TiO2 films. It will be demonstrated that oxygen rf discharge during the PLD process makes incorporation of oxygen into the depositing films extremely effective. This effect of the rf discharge allows operation of the PLD process at a lower oxygen background pressure while enhancing the deposition rate. Also, the production of a good quality TiO2 film by PAPLD using a pure metal titanium target will be shown.

Nanoparticle-Enhanced Laser Induced Breakdown Spectroscopy for the noninvasive analysis of transparent samples and gemstones

In this paper, Nanoparticle-Enhanced Laser Induced Breakdown Spectroscopy is applied to transparent samples and gemstones with the aim to overcome the laser induced damage on the sample. We propose to deposit a layer of AuNPs on the sample surface by drying a colloidal solution before ablating the sample with a 532 nm pulsed laser beam. This procedure ensures that the most significant fraction of the beam, being in resonance with the AuNP surface plasmon, is mainly absorbed by the NP layer, which in turn results the breakdown to be induced on NPs rather than on the sample itself. The fast explosion of the NPs and the plasma induction allow the ablation and the transfer in the plasma phase of the portion of sample surface where the NPs were placed. The employed AuNPs are prepared in milliQ water without the use of any chemical stabilizers by Pulsed Laser Ablation in Liquids (PLAL), in order to obtain a strict control of composition and impurities, and to limit possible spectral interferences (except from Au emission lines). Therefore with this technique it is possible to obtain, together with the emission signal of Au (coming from atomized NPs), the emission spectrum of the sample, by limiting or avoiding the direct interaction of the laser pulse with the sample itself. This approach is extremely useful for the elemental analysis by laser ablation of high refractive index samples, where the laser pulse on an untreated surface can otherwise penetrate inside the sample, generate breakdown events below the superficial layer, and consequently cause cracks and other damage. The results obtained with NELIBS on high refractive index samples like glasses, tourmaline, aquamarine and ruby are very promising, and demonstrate the potentiality of this approach for precious gemstones analysis.

Experimental and Theoretical Investigation of Nonequilibrium in Laser Induced Plasmas

Expansion of the material ablated from a TiO (titanium oxide) surface with a nanosecond pulsed laser is studied by emission spectroscopy. The quantities measured are the time of flight and a space resolved spectrum. The Boltzmann plot approach is used to obtain temperatures and concentrations of Ti (titanium) atoms and ions. A semi-empirical collisional-radiative model is shown to be very useful to eliminate spurious effects from the spectrum. A self-consistent model coupling collisional/radiative kinetics and fluid dynamic equations of the plume expansion can improve the understanding of the physics during the expansion ad can be used as a powerful tool to extract quantitative information from the emission spectrum even in the absence of LTE (local thermodynamic equilibrium).